In general, a single-phase induction motor includes a stator, on which a main coil and an auxiliary coil are wound with a spatial interval of 90 degrees therebetween. A source voltage is applied directly to the main coil, and also, is applied to the auxiliary coil via a capacitor and a switch. This is because the main coil cannot generate a starting force alone even if a voltage is applied thereto. Therefore, with the assistance of a start device such as the auxiliary coil, etc., the stator generates a rotating magnetic field to start the rotor.
The start device is classified, according to the kind thereof, into a split-phase type, shading-coil type, capacitor-start type, repulsive-start type, and the like.
As an example of a single-phase induction motor having the above-described start device, a capacitor-start type single-phase induction motor is illustrated in FIGS. 1 and 2.
FIG. 1 illustrates a stator and a rotor of a general single-phase induction motor, and FIG. 2 illustrates a simplified circuit of the rotor and stator coils.
When only a main coil 12 is wound on a stator 10, the stator 10 generates only an alternating magnetic field, and thus, starting of a rotor 20 is not accomplished. For this reason, an auxiliary coil 14 must be wound on the stator 10 to generate a rotating magnetic field. With the rotating magnetic field, the rotor can be started and rotated in a predetermined direction. Namely, starting torque arises via the rotating magnetic field.
In the simplified circuit of FIG. 2, a capacitor 15 serves to delay a phase of current to be applied to the auxiliary coil 14, so as to generate starting torque via interaction between the auxiliary coil 14 and the main coil 12. Once being started, rotation of the rotor 20 is maintained, under an assumption of no load variation, even if power is not applied to the auxiliary coil 14. Namely, it is unnecessary to apply power to the auxiliary coil 14 if the rotor 20 maintains a predetermined or more revolutions per minute after being started. However, when load varies, additional starting torque is required. Therefore, it is preferable that power always be supplied to the auxiliary coil 14 via the capacitor 15.
Of course, a three-phase induction motor can generate a rotating magnetic field even when only a main coil is wound on a stator, and does not require winding of the above-described auxiliary coil on the stator. That is, the three-phase induction motor does not require a separate start device.
An advantage of the above-described single-phase induction motor is that it does not require an inverter like a brushless DC (BLDC) motor or reluctance motor and can be started directly using single-phase commercial power, resulting in superior price competitiveness.
Now, the above-described general single-phase induction motor will be described in detail with reference to FIGS. 1 and 2.
The stator 10, which has a hollow internal configuration, includes a plurality of teeth 11 arranged, along an inner circumference thereof by a desired angular interval, to protrude radially inward, and a main coil 12 wound on the respective teeth 11 to have a polarity of N-pole or S-pole upon primary application of a current.
Here, an insulator (not shown) is interposed between the teeth 11 and the main coil 12. The insulator serves not only to provide electrical insulation between the teeth 11 and the main coil 12, but also to facilitate easy winding of the main coil 12.
The stator 10 further includes an auxiliary coil 14, which is wound on the stator 10 to have a desired spatial angular interval with the main coil 12 and is adapted to generate a rotating magnetic field upon application of a current. Of course, the auxiliary coil 14 is also wound on the teeth 11 by interposing an insulator therebetween. The main coil 12 and auxiliary coil 14 can be called together “stator coils” or simply “coils”.
The coils 12 and 14 are connected to a single-phase power source, and at the same time, are connected in parallel to each other. The capacitor 15 is connected to the auxiliary coil 14 in series. Again, although not shown, the capacitor 15 may be optionally connected to the power source via a switch.
Generally, the rotor 20 may be a frequently used squirrel cage rotor. FIGS. 1 and 2 illustrate the squirrel cage rotor.
The rotor 20 is normally fabricated by stacking steel plates one above another to constitute a rotor core, and each steel plate is formed, along an outer circumference thereof, with a plurality of slots 21 at desired radial positions from the center of the steel plate by a desired angular interval. The rotor 20 further includes rod-shaped conductive bars 22 inserted into the respective slots 21 of the rotor core. The rod-shaped conductive bars 22 are normally made of copper or aluminum rods.
Both ends of the squirrel cage rotor core are connected with not-shown end-rings (See FIGS. 13 and 14), to achieve electric connection via the conductive bars 22, and the end-rings are generally formed via aluminum die-casting. Specifically, the conductive bars 22 and end-rings are integrally formed with each other via aluminum the casting, and both the end-rings are located, respectively, at the top and bottom of the rotor core.
Meanwhile, the rotor 20 is centrally provided with a shaft hole 24. A rotating shaft (not shown) to transmit a rotating force of the rotor 20 to an external component will be press-fitted into the shaft hole 24, such that the rotor 20 and rotating shaft constitute a unitary rotating body.
In operation of the above-described single-phase induction motor, if power is applied to the coils, an induction current is generated in the conductive bars 22, causing the rotor 20 to be rotated by resulting induction torque. However, in this case, the conductive bars 22 cause loss, namely, conductive bar loss. Due to the conductive bar loss, therefore, improvement in efficiency of a motor of a predetermined size is limited. The single-phase induction motor is problematic when high-efficiency is required.
Other problems of the conductive bar loss include an increase in the temperature of the rotor 20 and a great variation of loss depending on the temperature variation. In particular, the higher the temperature, the greater the conductive bar loss. For this reason, improvement in motor efficiency at high temperature is limited.
Meanwhile, the single-phase induction motor, in consideration of characteristics thereof, must always be operated at a lower speed than synchronous speed, in order to generate induction torque. This is because the single-phase induction motor theoretically has zero torque at the synchronous speed, and the smaller the rotating speed, the greater the torque.
Accordingly, in the single-phase induction motor, as motor load, namely, load applied to the rotating shaft varies, a rotating speed of the rotating shaft, namely, a rotating speed of the motor varies, and this makes it difficult to control the motor depending in consideration of load variation.